With respect to fiber, a “composite” is often defined as a material consisting of 30%–70% fiber and 70%–30% matrix (Bolton 1995). However, in North America particleboards and fiberboards, which generally contain less than 10% adhesive or matrix, are sometimes referred to as composites. This section addresses plastic-type composites. In plastics, fibers are introduced to improve physical properties such as stiffness, impact resistance, bending and tensile strength. Man-made fibers of glass, kevlar and carbon are most commonly used today, but plant fibers offer considerable cost savings along with comparable strength properties.
“Geotextiles” or “agricultural textiles” include (1) ground-retaining, biodegradable matting designed to prevent soil erosion, especially to stabilize new plantings while they develop root systems along steep highway banks to prevent soil slippage (Fig. 32); and (2) ground-covers designed to reduce weeds in planting beds (in the manner of plastic mulch). At present the main materials used are polymeric (polythene, spun-blown polypropylene) and some glass fiber and natural fibers. Both woven and non-woven fibers can be applied to geotextiles; woven and knitted materials are stronger and the open structure may be advantageous (e.g. in allowing plants to grow through), but non-wovens are cheaper and better at suppressing weeds. Flax and hemp fibers exposed to water and soil have been claimed to disintegrate rapidly over the course of a few months, which would make them unacceptable for products that need to have long-term stability when exposed to water and oil. Coco (coir) fiber has been said to be much more suitable, due to higher lignin content (40%–50%, compared to 2%–5% in bast fibers); these are much cheaper than flax and hemp fibers (Karus et al. 2000). However, this analysis does not do justice to the developing hemp geotextile market. Production of hemp erosion control mats is continuing in both Europe and Canada. Given the reputation for rot resistance of hemp canvas and rope, it seems probable that ground matting is a legitimate use. Moreover, the ability to last outdoors for many years is frequently undesirable in geotextiles. For example, the widespread current use of plastic netting to reinforce grass sod is quite objectionable, the plastic persisting for many years and interfering with lawn care. Related to geotextile applications is the possibility of using hemp fiber as a planting substrate (biodegradable pots and blocks for plants), and as biodegradable twine to replace plastic ties used to attach plants to supporting poles. Still another consideration is the “green ideal” of producing locally for local needs; by this credo, hemp is preferable in temperate regions to the use of tropical fibers, which need to be imported.
Based on studies of sex reversal in hemp, it was first reported by K. Hirata in 1924 that an XY sex-determination system is present. At the time, the XY system was the only known system of sex determination. The X:A system was first described in Drosophila spp in 1925. Soon thereafter, Schaffner disputed Hirata's interpretation, and published results from his own studies of sex reversal in hemp, concluding that an X:A system was in use and that furthermore sex was strongly influenced by environmental conditions.